Exploring the limits of spontaneous emission coupling is not only one of the central goals in the development of nanolasers, it is also highly relevant regarding future large-scale photonic integration requiring energy-efficient coherent light sources with a small footprint. Recent studies in this field have triggered a vivid debate on how to prove and interpret lasing in the high-β regime. We investigate close-to-ideal spontaneous emission coupling in GaN nanobeam lasers grown on silicon. Such nanobeam cavities allow for efficient funneling of spontaneous emission from the quantum well gain material into the laser mode. By performing a comprehensive optical and quantum-optical characterization, supported by microscopic modeling of the nanolasers, we identify high-β lasing at room temperature and show a lasing transition in the absence of a threshold nonlinearity at 156 K. This peculiar characteristic is explained in terms of a temperature and excitation power-dependent interplay between zero-dimensional and two-dimensional gain contributions.

The realization of high-β lasers is one of the prime applications of cavity-QED promising ultra-low thresholds, integrability and reduced power consumption in the field of green photonics. In such nanolasers spontaneous emission can play a central role even above the threshold. By going beyond rate-equation approaches, we revisit the definition of a laser threshold in terms of the input-output characteristics and the degree of coherence of the emission. We demonstrate that there are new regimes of cavity-QED lasing, realized e.g. in high-Q nanolasers with extended gain material, for which the two can differ significantly such that coherence is reached at much higher pump powers than required to observe the thresholdlike intensity jump. Against the common perception, such devices do not benefit from high-β factors in terms of power reduction, as a significant amount of stimulated emission is required to quieten the spontaneous emission noise. arXiv:1809.08976v2 [cond-mat.mes-hall]

We investigate the relation between indistinguishability and quantum entanglement in Hong-Ou-Mandel-interference experiments theoretically and relate these quantum mechanical principles to the theorem of entanglement monogamy. We use Glauber's theory of quantum coherence to predict the detection statistics in HOM-interference of photons produced by three different photonpair sources: sources of spectrally indistinguishable frequency uncorrelated photons, sources of spectrally distinguishable frequency detuned photons and sources of spectrally indistinguishable frequency entangled photons. In the case of frequency entangled photon sources, we find a new additional term in the coincidence detection probability of HOM-interference compared to existing results, which is relevant for the case of low frequency separations or large single-photon bandwidths. Compared to previous work in that context we treat all photonic degrees of freedom (DOF) on equal footing.

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